A conventional map display device has a function of placing a character string, such as a road name, from a point where the placement of the character string is started (referred to as a character string placement start point from here on) along a road line (referred to as a road node series from here on) (for example, refer to FIG. 21). In this case, when the character string placement start point is not appropriate, an overlap between the character string and another character string as shown in FIG. 22(a), a reduction in the readability due to a change in the display angle of a character as shown in FIG. 22(b), and a reduction in the readability due to a high degree of closeness between the character string and another character string as shown in FIG. 22(c) take place.
As a method of solving such an overlap between character strings as shown in FIG. 22(a), and such a reduction in the readability as shown in FIG. 22(b), for example, a method of changing a character string placement start point to a position having high readability is described in nonpatent reference 1. Concretely, the method is the one of changing a character string placement start point by minimizing an evaluation function f shown by the following equation (1) and consisting of both a function overlap(i) showing an overlap between character strings as shown in FIG. 23(a) and a function flatness(i) showing a variation in the angle of a road node series along which a character string is placed as shown in FIG. 23(b). α1 and α2 in the equation (1) are parameters for adjusting the values of overlap(i) and flatness(i) respectively, and String_num shows the number of character strings.
                    f        =                                            ∑              i              String_num                        ⁢                                                  ⁢                                          α                1                            ⁢                              overlap                ⁡                                  (                  i                  )                                                              +                                    α              2                        ⁢            flatness            ⁢                                                  ⁢                          (              i              )                                                          (        1        )            